This invention relates to a thermally responsive switch which is used as a thermal protector for protecting electric motors or the like against overcurrent and overheating conditions and includes a bimetallic or trimetallic thermally responsive element having a dish-shaped portion which reverses its curvature with a snap action in response to heat and a pair of contacts for making and breaking an electrical circuit upon the snap action of the thermally responsive element.
A motor protecting thermally responsive switch of the above-mentioned type is disclosed in Japanese Published Patent Application (Kokoku) No. 49-24744 corresponding to U.S. Pat. No. 3,538,478. This publication does not disclose the operating temperature calibrating mechanism for adjusting a contact pressure between a movable contact secured to a moving end of a bimetallic thermally responsive element and a fixed contact secured to a contact arm. Furthermore, the thermally responsive element is positioned in an assembly by way of welding so that the contact pressure necessary for calibrating the operating temperature of the element is in a predetermined range. In the case where the thermally responsive element is not suitably positioned in the assembly, there is no way to suitably position it except by bending the contact arm to which the fixed contact is secured. However, such bending causes a spring back action, resulting in variations in the operating characteristics of the switches.
Japanese Laid-open Patent Application (Kokai) No. 62-88232 corresponding to U.S. Pat. No. 4,672,353 assigned to the same assignee as the present application discloses a snap-action type thermally responsive switch provided with a calibrating mechanism. In this calibrating mechanism, a first receiving portion engages the generally central portion of a thermally responsive element. One end of the thermally responsive element is secured to a strip which is further secured to a second receiving portion of a support for supporting the thermally responsive element. The thermally responsive element carries at the other end a movable contact which is engaged with a fixed contact. An elastic plate is provided between the thermally responsive element and the support. Thus, the construction of the thermally responsive switch disclosed is complicated. Accordingly, when an overcurrent sufficient to operate the thermally responsive switch flows in the circuit and a sufficiently high temperature to reverse the thermally responsive element is reached, a series combination of the elastic plate and thermally responsive element is connected in parallel to the calibration mechanism such that the current is caused to flow into a bypass circuit between the first receiving portion contacting the approximately central portion of the thermally responsive element and the distal end of the thermally responsive element to which end the movable contact is fixed. Furthermore, since the contact pressure between the first receiving portion and the approximately central portion of the thermally responsive element changes in the increasing temperature stage of the thermally responsive element, the resistance of the bypass circuit also varies. Consequently, since the value of composite resistance of the internal circuit of the thermally responsive switch varies, the current versus operating time characteristic of the switches differs from product to product. The current versus operating time characteristic here refers to the characteristic in accordance with which the thermally responsive switch must operate to protect the motor or the like within a predetermined time against overcurrent of a preselected value in the same atmosphere. For example, when the reference response time is 10 seconds in the case where the current of 30 amperes flows at 25.degree. C., the response time is 8 seconds for one occasion and 15 seconds for another occasion even in the same product. Particularly, the variations in the current versus operating time characteristic become increased in case of mass production of the thermally responsive switches, resulting in inconsistency as a protecting device. In order to prevent such an inconsistency, a mechanism is necessary for electrically insulating the contact portion where the first receiving portion contacts the central portion of the thermally responsive element and the contact portion where the second receiving portion contacts the connecting strip secured to one end of the thermally responsive element, which complicates the construction of the thermally responsive switch.